Vanilla 1 is no longer supported or maintained. If you need a copy, you can get it here
Make a guest user account and define the username and password in the default.php file and it should just add the add comments form to all pages and allow guests to post. It will log them in and immediately back out again in order to do so. I suggest you choose a hard to guess password since otherwise someone will just log into the guest account properly, change its password, and break the extension. If people are posting while not logged in it will add a warning that they are posting as guest.
The extension will not work in conjunction with the AddComments extension since that requires the fields to be filled in (if you're php savvy you could just remove that requirement from the addcomments/default.php and it should work fine)
As of v1.3, if a guest enters their own name into the Username box and leaves the password box free, it will head up the post with their name. This requires the HTML formatter to be installed.
Let me know how it goes.